Tiffany is 4 times as old as Ashley and is also 21 years older than Ashley. How old is Tiffany?
Answer: We can use the given information to write down two equations that describe the ages of Tiffany and Ashley. Let Tiffany's current age be $t$ and Ashley's current age be $a$ $t = 4a$ $t = a + 21$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $t$ is to solve the second equation for $a$ and substitute that value into the first equation. Solving our second equation for $a$ , we get: $a = t - 21$ . Substituting this into our first equation, we get the equation: $t = 4$ $(t - 21)$ which combines the information about $t$ from both of our original equations. Simplifying the right side of this equation, we get: $t = 4t - 84$ Solving for $t$ , we get: $3 t = 84$ $t = 28$.